But is vitamin D truly a cure-all? And if the benefits are real, should we all be taking vitamin D supplements or even fortifying our foods?

Vitamin D is not one chemical, but a label that covers a group of substances, including vitamin D2 and D3. The latter is the form made when sunlight hits your skin and is also found in other animals. Non-animal sources such as fungi and yeasts primarily produce the D2 form. Once in the body, these substances are converted into biologically active steroids that circulate in the blood.

One area where the impact on health appears to be clear is vitamin D’s role in keeping bones and teeth healthy and improving muscle strength.

“The musculoskeletal stuff is really good and really strong,” said Helen Bond, a spokesperson for the British Dietetic Association, pointing out that vitamin D is important in calcium and phosphate absorption.

Too little vitamin D can be serious: the skeletal disorders osteomalacia and ricketsare known to be caused by a vitamin D deficiency, and the latter is on the rise in the UK, a finding some put down to the impact of poverty on poor nutrition.

But do the wider health claims stand up?

“Intuition suggests that it can’t all be right,” said Julia Newton-Bishop, professor of dermatology and vitamin D expert from the University of Leeds. But while a recent review of evidence by the scientific advisory committee on nutrition only found strong evidence in the case of bone and muscle health, Newton-Bishop says a growing body of research is exploring other conditions.

Newton-Bishop says the fact that receptors for vitamin D are present on a huge array of body cells suggests the substance might indeed play a central role in our health, adding that human history offers further evidence: as humans moved to higher latitudes, skin tone became paler. “[One] explanation is that vitamin D was so important that that was a selective pressure,” she said. “The fact that Inuits aren’t pale-skinned and for millennia they have had an exclusively fish diet is an argument for the fact that vitamin D was a driver, because why would they be different to everyone else?”

Martin Hewison, professor of molecular endocrinology at the University of Birmingham, who carried out the recent study into vitamin D and rheumatoid arthritis, said evidence from cell studies backs up the idea that the vitamin is important.

“In most of the models, vitamin D appears to have quite a positive effect,” he said. “If you are using cancer cell lines or cancer cells, vitamin D has anti-cancer effects, and likewise in cells that have been used for models for infection and immune disorders, vitamin D has quite clear antibacterial and anti-inflammatory effects.”

But when it comes to studies in humans, the picture is far from clear-cut. While some studies find links to diseases, others do not.

That, say experts, could be partly down to the way they are conducted – for example, not all studies take into account the starting levels of vitamin D in participants, or they may have been carried out in populations with different genetic factors that might affect the impact of vitamin D.

Other experts have doubts about vitamin D’s influence. Prof Tim Spector, author of The Diet Myth, wrote in the Independent: “The evidence so far suggests (with the possible exception of multiple sclerosis and some cancers) that low vitamin D levels are either irrelevant or merely a marker of the disease.”

Hewison says that while vitamin D might help prevent certain conditions such as tuberculosis, respiratory infections and autoimmune diseases,it should not be seen as a cure for them. “It is good at protecting against things,” he said, “but once a disease is settled in, it is unlikely you are going to be able to give somebody who has got prostate cancer vitamin D and it is going to get dramatically better.”

What about the case for supplements? With some having previously been found to cause more harm than good, Newton-Bishop says caution towards this apparent panacea is unsurprising. “Everyone within the cancer world is nervous about supplements,” she said. “I would say to patients don’t take supplements, with the exception of avoiding a low vitamin D level.”

But how low is low? With the amount of sunlight needed varying with genetics, skin colour, time of day, how much one covers up and a host of other factors, the scientific advisory committee on nutrition said it was too difficult to say how much sun we need to make sure our vitamin D levels are up to scratch. In any case, from October until March the sun in the UK isn’t strong enough to do the job.

“There are very few naturally rich sources of vitamin D, and most really good sources are of animal origin, which doesn’t bode well for vegans and vegetarians,” she said. “A serving of oily fish like mackerel will give you easily your 10 micrograms of vitamin D a day, but if you drop down to a tin of canned tuna, you are only getting 1.5 micrograms.”

And as Adrian Martineau, clinical professor of respiratory infection and immunity at Queen Mary University of London, points out, even in the summer, “sunshine isn’t going to be the answer, especially because there is an associated risk of skin cancer.”

If you are considering taking supplements, it might be worth checking which form of vitamin D they contain. “Some people don’t want an animal form of vitamin D,” said Hewison. However, “What studies have shown is that if you want to raise your blood vitamin D levels, vitamin D3 is much more efficient at doing that.”

Dr Benjamin Jacobs, a consultant paediatrician and spokesperson for the Royal College of Paediatrics and Child Health, says supplements are not enough as it is hard to make sure people actually take them. Instead, he suggests the UK consider food fortification.

Some countries, including Canada and Finland, have embraced fortification of milk. But although infant formula and some breakfast cereals, plant-based milks and fruit juices are already fortified in the UK, most foods are not.

Hewison believes the government should consider a national fortification plan and that the risks of it resulting in dangerously high vitamin D intake are negligible: “I think most people in the field agree that if you want to have a large-scale improvement in people’s vitamin D levels then it can only really be done through fortified foods.”